It is known to monitor road intersections controlled by traffic lights. To this end a sensor is arranged on the lane in the range of the traffic light, which sensor supplies a sensor signal when a vehicle passes over it. If such a signal appears during the stop interval of the traffic light, that is if a vehicle enters the road intersection during the stop interval contrary to traffic regulations, a photographic camera will be triggered, the field of view of which covers the road intersection controlled by traffic lights. Thereby a photograph is taken of the traffic violation and also of the license plate of the vehicle in question (DE-A- 683,658).
It is also known to trigger by the sensor signal successively several photographic exposures in adjustable time intervals, if a sensor arrangement establishes a traffic violation of a vehicle. This traffic violation may consist in that the vehicle enters a road intersection during the stop interval. Then the photographic camera is triggered by a signal when passing a contact sensor embedded in the lane. The traffic violation may also consist in that the vehicle runs too fast. In this case the photographic camera is triggered by two contact thresholds embedded one behind the other at a fixed distance in the lane, being passed within a predetermined time interval corresponding to the speed limit (DE-A- 10 78 797 or AT-A- 225,077). The two contact sensors embedded one behind the other in driving direction exclusively serve for the speed measurement.
By DE-C- 2,365,331 a device is known for photographic monitoring of road intersections controlled by traffic lights, which device has a photographic camera covering the road intersection and a sensor responding to a vehicle entering the road intersection, and supplies a sensor signal by means of a detector circuit. A camera-triggering device is aranged to be energized by a first triggering signal triggered by the sensor signal through a first gate circuit for triggering a first exposure. The first gate circuit is arranged to be rendered conductive by a signal from the traffic light during its stop interval. When the first exposure is triggered, a time function element is triggered and changes its state for a predetermined delay time. After expiration of the delay time, a second triggering signal applied to the camera-triggering device for triggering an exposure is produced by the time function element. The first gate circuit is then non-conductive from the triggering of the first exposure to the end of the second exposure. In this manner a second vehicle entering the road intersection shortly after the first triggering vehicle cannot directly cause a triggering signal directly, before the cycle consisting in the first and the second exposures initiated by the first vehicle is finished. The sensor signal is, however, simultaneously applied to a second gate circuit which is rendered conductive during the non-conductive period of the first gate circuit. The outlet of this second gate circuit is also applied to the camera-triggering device through a signal-retarding time function element having a delay time which is longer than the time required for the two exposures. The sensor pulse of the second vehicle is thus, so to say, memorized until the first two exposures inclusive the film transport are certainly finished. Then a third exposure is made. The second vehicle is then certainly visible on one of the two exposures triggered by the first vehicle and supplementary on the third exposure, such that two exposures are also made of the second vehicle. This device is also described in the U.S. Pat. No. 3,858,223.
The U.S. Pat. No. 3,858,223, as well as the DE-B- 2,307,217 furthermore show a device for photographic monitoring road intersections controlled by traffic lights, in which a vehicle, when entering the road intersection, successively triggers two exposures. A second sensor is provided in a lane transverse to the lane to be monitored, by which sensor the cameratriggering device may be energized through a gate circuit which is rendered conductive during the delay time between the first and second exposures. Thereby vehicles driving fast are detected, which enter the road intersection in the stop interval and turn off immediately in a line crossing the non authorized road before the delay time of the time function element expires, and the second exposure is triggered. In this case an additional exposure is triggered through the second sensor before expiration of the predetermined delay time. This exposure then covers the vehicle turning off. In this device two sensors are located in two intersecting lines. The employment of the second sensor (contact sensor or induction loop) has the specific purpose to detect vehicles turning off. Vehicles driving straight on do not cause response of the second sensor and are not covered by the exposure triggered after a predetermined delay time.
Cameras for photographic monitoring of road intersections controlled by traffic lights are furthermore described in the U.S. Pat. No. 2 871 088, The U.S. Pat. No. 3,849,784 and the U.S. Pat. No. 3,866,165.
In the devices of the prior art mentioned above, the vehicles are photographed from the rear, the back license plate appearing on the photographic. The driver of the vehicle is, in general, not recognizable on such photographs. Therefore it is known (DE-A- 2,129,984 and DE-C- 2,802,448) to arrange the camera such that a so called "frontal photographing" takes place, thus the vehicles are photographed from the front opposite to the driving direction. Such a frontal photographic, if it is made with flash illumination, requires special precautions in order to prevent dazzling of the driver, and furthermore to make sure that the driver can be recognized through the frontal windshield of the vehicle. Therefore a red filter is conventionally arranged before flash and camera and a red-sensitive film is used.
The devices of the prior art in which two exposures made in a fixed time interval are triggered successively by a vehicle entering the road intersection during the stop interval, still present certain disadvantages:
It may happen that a vehicle driving very fast has driven straight on out of the field of image during the delay time between first and second exposures. With photographing from the rear the vehicle is then at such a distance that its license plate cannot be recognized any more on the second exposure, the evidence value of the second exposure becoming doubtful. With frontal photographing it may happen, that a vehicle driving fast has already passed the camera during the delay time and is not covered any more at all. Vehicles passing a road intersection at high speed during the stop interval represent, however, a particular source of danger. Therefore it is important to include especially such vehicles.
But also with slowly driving vehicles, the devices of the prior art may fail. When the vehicle at low speed has caused the sensor to respond and thus the delay time starts to run, it may happen that at the moment of the second exposure this vehicle is not yet in the danger area. Then the vehicle can continue to drive over the road intersection without the authorities being able to prove this from the exposures.
Finally, in the devices of the prior art, the distance of the photographed vehicle from the camera depends on the speed of the vehicle. This is particularly disadvantageous with frontal photographing, where it is necessary to identify a person behind the windshield on the photograph under frequently unfavourable illumination conditions.